1. Field of the Invention
The present invention relates to a digital signal reproducing apparatus, and more particularly to an apparatus for reproducing an encoded digital signal which decodes an encoded data signal, on the basis of a frame synchronization signal, from a read signal derived by playing a recording medium on which the frame synchronization signal is recorded as encoded digital signals together with the encoded data signal.
2. Description of the Related Art
For correctly reproducing a digital signal from a read signal by playing a recording medium on which encoded digital signals are recorded, it is known that the frame synchronization must be obtained. For the frame synchronization, a synchronization pattern consisting of a code train, which does not exist in any code train patterns of data signals modulated for recording, is recorded at the head of each frame, and the synchronization pattern is detected as a frame synchronization signal from a read signal upon reproduction and used as the basis for reading and decoding data signals.
For example, in an optical disc which employs a (1, 7) modulation as a modulation scheme, data patterns are created with a minimal inversion interval being defined to be 2T (T is a bit interval) and a maximal inversion interval to be 8T. The frame synchronization pattern may be selected to be a pattern composed of repetition of maximal inversion intervals which will never appear in the data patterns. In an optical disc of a sampled servo system, the frame synchronization pattern is defined to have a length equal to a maximal inversion interval plus 1T.
It can be thought that a reproducing apparatus for reproducing a digital signal on the basis of a read signal from a recording medium on which the frame synchronization pattern as described above is recorded together with data patterns is arranged, for example, as shown in FIG. 1. In this reproducing apparatus, an RF (Radio Frequency) signal, which is a read signal output from a pickup (not shown), is supplied to a binary-coding circuit 11. The binary-coding circuit 11 generates a binary-coded signal by comparing an RF signal having a waveform as shown in FIG. 2A with a slice level V.sub.TH used as a threshold. The binary-coded signal having a waveform as shown in FIG. 2B is supplied to a PLL (Phase Locked Loop) circuit 12 as well as to a sampling circuit 13. The PLL circuit 12 generates a clock signal (FIG. 2C) which is synchronized with edges of the binary-coded signal, such that the sampling circuit 13 performs a sampling operation in response to the clock signal in order to generate a signal having a waveform as shown in FIG. 2D. The output signal of the sampling circuit 13 is supplied to a pattern detector circuit 14 and a data decoder circuit 15. The pattern detector circuit 14, as shown in FIG. 3, comprises a shift register 14a which receives and holds a sampled signal bit by bit in synchronism with the clock signal, a pattern memory 14b for previously recording the synchronization pattern signal derived by the frame synchronization pattern, and a comparator circuit 14c for generating a pattern detection signal when a signal held in the shift register 14a is coincident with the synchronization pattern signal output from the pattern memory 14b.
The pattern detection signal output from the pattern detector circuit 14 is supplied to a timing generator circuit 16 which generates a timing signal to a decoder circuit 15 in response to the pattern detection signal. The decoder circuit 15 decodes digital data from an output signal of the sampling circuit 13 in accordance with the timing signal and the clock signal.
It should be noted that the respective waveforms shown in FIG. 2A-2D are based on a synchronization pattern signal composed of "1" for 8T and the next "0" for 8T, where 8T is a maximal inversion interval.
FIG. 4 shows an exemplary arrangement of the pattern detector circuit 14 which comprises a shift register 14A and an AND circuit 14B.
In the reproducing apparatus which detects the frame synchronization signal from a binary-coded waveform as described above, although an RF signal is obtained as a waveform which relatively abruptly changes as shown in FIG. 5A when a recording density on a recording medium is low, the RF signal is derived as a waveform which slowly changes as shown in FIG. 5B when the minimal inversion interval is narrow and hence the recording density on the recording medium is high. In the latter case, if the slice level is offset or if large noise is introduced into the slice level, the frame synchronization signal is more susceptible to erroneous detection. FIG. 5C shows an example in which an RF signal read from a high-density recording medium is output from the binary-coding circuit as a binary-coded signal composed of 8T of "1" and 8T of "0", which however should be read as being composed of 2T of "0", 3T of "1", and 8T of "0" in this order. In the event, the frame synchronization signal is erroneously detected.